Diffractive monitor and system

ABSTRACT

A monitor for a light beam creates a monitor beam by deflecting a portion of the application beam and further manipulating the monitor beam and/or the application beam to allow more efficient use thereof. For example, the monitor beam may be collimated to allow an increase in spacing between the device outputting the light beam and a detector for sensing the monitor beam. Alternatively or additionally, the monitor beam may be focused to allow use of a smaller detector and of a smaller percentage of the application beam. The diffractive element deflecting the beam may be either transmissive or reflective. The additionally manipulation of the monitor beam and/or the application beam may be provided by the same diffractive element which deflects the beam, which is particularly useful when the diffractive element is reflective, and/or by additional optical elements.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.09/984,915 filed Oct. 31, 2001, now U.S. Pat. No. 6,404,959 issued Jun.11, 2002, which is a continuation of U.S. application Ser. No.09/386,280 filed Aug. 31, 1999, now U.S. Pat. No. 6,314,223 issued Nov.6, 2001, which claims priority under 35 U.S.C. §119 to U.S. ProvisionalApplication No. 60/097,830 filed on Aug. 31, 1998, the entire contentsof all of which are hereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to monitoring a light beam,specifically for a diffractive monitor, particularly for use in afunctional or application beam path.

2. Description of Related Art

Light emitting devices such as VCSELs need some form of power control tomaintain a constant output. Such control is typically automaticallyperformed by measuring an output of a light emitting device duringoperation and using this measurement to control the power supplied tothe light emitting device.

Such control may be easily achieved when the light emitting device to becontrolled is an edge emitting laser because edge emitting lasers outputlight from two ends thereof. Thus, one output may be used for thedesired application, while the other output may be used for the powercontrol.

In contrast, a VCSEL typically only emits light from one surface. Hence,any monitoring of the light must be from the same output as used for thedesired application of the VCSEL. VCSELs are much cheaper and theirsurface emissions make them easier to integrate with other opticaldevices than the edge emitting lasers, so the use of VCSELs is verydesirable.

Previous attempts to monitor the power of VCSELS typically involvesplitting off of a portion of the output beam to use as a monitor beam.Examples of such configurations are disclosed in U.S. Pat. Nos.5,757,836 and 5,774,486. However, such splitting off obscures part ofthe beam which may affect the wavefront and imaging, and hence coupling,of the light. Further, if the intensity distribution changes, such aswhen there is a change in lasing mode, the monitored power may change ina way which does not represent the overall output power of the VCSEL.

Additionally, splitting off of the beam may require the power of a lightbeam to be increased in order to maintain the requisite power levelwhile allowing detection. Previous uses of scattering the beam to createa monitor beam relied on reflection for directing the beam and did notprovide an optimal signal to the monitor detector. Further, previousscattering did not insure the entire beam was being monitored.

SUMMARY OF THE INVENTION

The present invention is therefore directed to monitoring a light beamwhich substantially overcomes one or more of the problems due to thelimitations and disadvantages of the related art.

These and other object may be realized by monitoring a light beam to beused in an application. Such monitoring may be performed using adiffractive to separate a percentage of the beam to be used to form amonitor beam and detecting the monitor beam.

These and other objects of the present invention will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating the preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will bedescribed with reference to the drawings, in which:

FIGS. 1a and 1 b are side views of an embodiment of the presentinvention incorporating a reflection diffractive element for forming amonitor beam;

FIG. 2 is a side view of another embodiment of the present inventionincorporating a collimating transmission diffractive element for forminga monitor beam;

FIGS. 3a and 3 b are side view of systems incorporating the powermonitor of the present invention;

FIG. 4 is a side view of an embodiment of the present inventionincorporated with a can housing the light emitting device;

FIG. 5 is a side view of another embodiment of the present inventionincorporating a collimating transmission diffractive element for forminga monitor beam; and

FIG. 6 is a side view of a power monitor of the present invention inconjunction with an array of light emitting devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

A configuration for monitoring power of an entire beam is shown in FIG.1a. A device 10, e.g., a VCSEL or a light emitting diode, emits a lightbeam 15 to be used in a desired application. The light beam 15 impingesupon a diffractive element 20, preferably on a first surface 25 of asubstrate 30. The diffractive element 20 is preferably a shallowdiffractive structure, i.e., having phase depths of less than 2π, thedepth of which is determined by the wavelength of the light emittingdevice 10 and the desired efficiency of the diffractive element. Use ofa shallow diffractive structure reduces the amount of light diffractedinto the first order, i.e., into the monitor beam. Alternatively, thissame effect may be realized may varying the width of the step heights sothey are not equal to one another.

The diffractive element 20 transmits a majority of the beam into thezeroth or undiffracted order to form an application beam 40, whiletypically transferring a small percentage into other orders, including areflective order. In some embodiments, up to 50% of the light may bedeflected into higher orders to form the monitor beam. Light transferredinto the reflective order is across the entire light beam and forms amonitor beam 45. Since angles associated with a reflective order to bemonitored, typically the first reflected order, are known, anappropriately positioned monitor 50, e.g., a photodetector, measures thepower of the monitor beam 45 and the measured power is then used tocontrol the operation of the light emitting device 10 in a known manner.A surface 35 opposite the diffractive element may include additionaloptical elements 37, either diffractive, refractive or hybrid, forperforming further optical functions on the application beam 40, such ascollimating, focusing, and/or coupling the light into a fiber.

As an illustrative example, if the light 15 has a wavelength of 830 nm,a diffractive element having an eight level blazed grating made in fusedsilica was designed to have a total structure depth of approximately2300 Å. This depth was chosen so that approximately 90% of the light istransmitted into the zeroth order to form the application beam 40,approximately 2% of the light is reflected into the first reflectedorder to form the monitor beam 45, approximately 1.4% of the light istransmitted into the first transmission order, with the remainder of thelight being transferred to higher orders in decreasing percentages.

In both FIGS. 1a and 1 b, the diffractive element 20 also focuses themonitor beam onto the detector 50. The focusing of the monitor beamavoids VCSEL feedback encountered when a grating is used to simplyscatter the light. Further, the focusing allows the detector to besmaller. In general, it is preferable that the diffractive opticalelement used in the present invention provides optical functioning,e.g., collimation, focusing, to the monitor beam, particularly when thediffractive optical element reflects the monitor beam so no otheradditional surfaces are available on which to provide an optical elementfor further optical functioning to the monitor beam. Such a diffractiveelement is preferably a computer generated hologram.

As shown in FIG. 1b, additionally or alternatively, a transmissiondiffractive element for performing at least one of the further opticalfunctions on the application beam 40 may be incorporated with thereflective diffractive element 20 to form a combined diffractive 22. Ascan be seen in FIG. 1b, the application beam 40 is converging afterleaving the first surface 25 of the substrate 30. The incorporation maybe achieved by multiplexing the transmission hologram for forming thetransmission diffractive element with the reflective hologram forforming the reflective diffractive element, discussed below.

For a transmission hologram, the physical step height d needed to get aphase depth of phi (φ) is:

d=λ*φ/{2π*(n−1)}

where n is the refractive index.

For a reflection hologram, the physical step height d needed to get aphase depth of phi (φ) is:

d=λ*φ/(2*2π)

To get a phase depth of 2π, the etch depth is λ/(n−1) for a transmissionhologram and λ/2 for a reflection hologram. Thus, depending on therefractive index of the material, the reflection hologram may be muchshallower than the transmission hologram for the same phase depth. Whencombining these two types of holograms, the transmittance functions forthe reflection hologram and transmission hologram are multiplied, i.e.,their phase functions are added together. However, the phase values forthe two types of holograms are encoded differently in accordance withthe above equations. For example, if the index n=1.5, then a phase depthof 2π for the transmission hologram is 2λ, four times greater than theλ/2 for the reflection hologram. If sixteen levels are used to encodethe transmission hologram, then the four shallowest levels willcorrespond to 0, π/2, π and 3π/2 for the reflection hologram.

When the phase functions are added for sufficiently low index materials,the reflection hologram function will only be a small modulation on thetransmission hologram function, since the depth of the reflectionhologram is much shallower, so there is not a large effect of onefunction degrading the other. Indeed, when using a plurality of levels,often the shallower levels needed for the transmission hologramcorrespond to many of the levels needed for the reflection hologram. Ofcourse, if needed, either one or both of the hologram could incorporatelevels having phase depths in excess of 2π.

An embodiment employing a transmission hologram for forming the monitorbeam is shown in FIG. 2. The light emitting device 10 again outputs abeam of light which is incident on a substrate 30 having a first surface25 and a second surface 35. The diffractive element 55 on the firstsurface deflects some light 60 off to the side. As can be seen in FIG.2, the diffractive element 55 also collimates the monitor beam. Thisadditional optical functioning may be realized by simply adding the twotransmission functions together. Such collimation insures more lightwill be delivered to the detector 50 and allows greater separationbetween the VCSEL and detector, since the monitor beam is no longerexpanding.

The deflected light 60 is then reflected off of the second surface bytotal internal reflection if the deflection angle is sufficiently steep,i.e., exceeds the critical angle at the second surface interface, or byan appropriately placed reflector on the second surface, such as a patchof metal 65. The reflected beam 70 then returns to the first surface,where it is incident on another transmission hologram 72 which focusesthe monitor beam 75 onto the detector 50.

Again, the second surface may include optical elements for performingfurther optical functioning on the application beam 40. In the exampleshown in FIG. 2, the further optical element 37 on the second surface 35of the substrate 30 focuses the application beam 40 in order to couplethe light into an optical fiber 78.

The another transmission hologram 72 for focusing the deflected lightonto the detector may be omitted if the diffractive element 55 isdesigned to provide focus to the deflected light beam. Such a designwould eliminate the additional loss incurred if the light passes throughanother diffractive optical element having power. Further, as shown inFIG. 2, the deflected light may be reflected a plurality of times if theangle is appropriate or if reflectors 65 are provided on the firstsurface of the substrate. This increases flexibility regarding placementof the detector. Further, as discussed above in conjunction with thereflection diffractive, additional optical elements may be incorporatedwith the transmission diffractive to provide further optical functioningon the application beam 40, either on the first surface or the secondsurface of the substrate 30.

The entire system may be integrated as shown, for example, in FIGS. 3aand 3 b. When the light emitting device is directly attached to a glasssubstrate as shown in FIG. 3a, the configuration shown in FIG. 2 ispreferred. In FIG. 3b, the light emitting device is attached to a secondsubstrate 80 which is separated from the substrate by spacer blocks 85,resulting in sufficient room to use either power monitor configuration.For either of these configurations, the integration may occur on a waferlevel.

As an alternative to the use of substrates, the reflection diffractiveelement may be incorporated into a plastic or glass cap 90 which may goinside a can 95 housing the light emitting device as shown in FIG. 4.The cap forms a protective, potentially hermetic, seal for the lightemitting device. The cap 85 may be made in many ways, includinginjection molding.

Another configuration is shown in FIG. 5. A shown therein, light fromthe VCSEL 15 impinges upon a diffractive element which both splits off aportion of the beam to form a monitor beam and collimates both themonitor beam and the application beam. Such collimation allows increasedflexibility in placement of the detector 10, since the monitor beam isno longer expanding. Additionally, the diffractive element 55 may alsobe used to focus the monitor beam on the detector 50. Alternatively, arefractive element may be provided on a surface adjacent the diffractiveelement 55 to provide the focusing. When placed near each other, i.e.,the elements are close enough that the deflection created by thediffractive element still allows the beam to be focused by the focusingelement, the relative order of the diffractive element and the focusingelement doesn't matter. Preferably, the two substrates used in FIG. 5are created and bonded together on a wafer level and them diced in orderto form the optical system for both the monitor beam and the main orapplication beam shown therein.

An example of a configuration for monitoring an array of VCSELs is shownin FIG. 6. A monitor detector is provided for at least one of theVCSELs. The at least one detector, typically a corresponding array ofdetectors, is positioned in the plane of the drawing and a front view ofeach VCSEL 10 with a corresponding detector 55 would look like FIG. 5.Again, the monitor beam may be collimated and/or focused by thediffractive element. This collimation and/or focusing is particularlyimportant when monitoring more than one beam of the array in order toinsure the monitor beams do not influence one another.

Another alternative to the use of substrates, when the light is to becoupled to a fiber, includes incorporating the diffractive element forforming the monitor beam in a rod attached to the fiber.

While typically the entire beam is to be used, if only part of the beamis used, the diffractive element may create the monitor beam only fromthe portion of the beam to be used in the application. For example, ifcoupling the light to a fiber, only the portion of the beam which willbe delivered to the core region of the fiber may be monitored.

Although preferred embodiments of the present invention have beendescribed in detail herein above, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptstaught herein, which may appear to those skilled in the art, will stillfall within the spirit and scope of the present invention as defined inthe appended claims and their equivalents. For example, the powermonitoring of the present invention may be used to monitor any lightbeam. Further, aspects other than power may be detected.

What is claimed is:
 1. A monitor for monitoring a characteristic of alight beam comprising: an optical system comprising at least twosurfaces, said optical system supplying a functional light beam to anapplication, the functional light beam passing through two of the atleast two surfaces; a diffractive optical element on one of said atleast two surfaces splitting off a percentage of the light beam tocreate a monitor beam; another optical element on one of said at leasttwo surfaces which performs further optical functioning on at least oneof the monitor beam and the functional light beam; and a detectorreceiving the monitor beam.
 2. The monitor of claim 1, wherein saidoptical system further comprises performing at least one opticalfunction to both the monitor beam and the functional light beam.
 3. Themonitor of claim 2, wherein said at least one optical function comprisesfocusing said functional beam into a fiber.
 4. The monitor of claim 1,wherein the diffractive optical element is a transmission diffractivedeflecting a percentage of the light beam to form a deflected beam. 5.The monitor of claim 1, wherein said another optical element comprisinga focusing optical element focusing the monitor beam onto the detector.6. The monitor of claim 4, further comprising metal on a surfaceopposite the transmission diffractive element, said metal reflecting themonitor beam.
 7. The monitor of claim 1, wherein the light beam is anarray of light beams and said diffractive optical element and saiddetector are provided for at least one light beam in the array.
 8. Themonitor of claim 1, wherein all elements of said optical system areintegrated onto a single substrate.
 9. A monitor for a light beamcomprising: a diffractive optical element creating a monitor beam fromthe light beam, the creating including both splitting off a percentageof the light beam and performing further optical functioning on thelight beam, a remaining light beam not split off being transmittedthrough the diffractive optical element; and a detector for receivingthe monitor beam.
 10. The monitor of claim 9, wherein the diffractiveoptical element is a reflective diffractive reflecting a percentage ofthe light beam to the detector.
 11. The monitor of claim 9, wherein thediffractive optical element is a transmission diffractive deflecting apercentage of the light beam to form a deflected beam.
 12. The monitorof claim 9, wherein the further optical functioning includes focusingthe monitor beam onto the detector.
 13. The monitor of claim 9, whereinthe further optical functioning includes collimating the monitor beam.14. The monitor of claim 9, wherein the light beam is an array of lightbeams and said diffractive optical element and said detector areprovided for at least one light beam in the array.
 15. A monitoringsystem comprising: a device outputting a light beam; an optical systemcomprising at least two surfaces, said optical system supplying afunctional light beam to an application, the functional light beampassing through two of the at least two surfaces; a diffractive opticalelement on one of said at least two surfaces splitting off a percentageof the light beam to create a monitor beam; another optical element onone of said at least two surfaces which performs further opticalfunctioning on at least one of the monitor beam and the functional lightbeam; and a detector receiving the monitor beam.
 16. The monitoringsystem of claim 15, wherein the diffractive optical element is areflective diffractive reflecting a percentage of the light beam to thedetector.
 17. The monitoring system of claim 15, wherein the diffractiveoptical element is a transmission diffractive deflecting a percentage ofthe light beam to form a deflected beam.
 18. The monitoring system ofclaim 15, wherein said another optical element focuses the monitor beamonto the detector.
 19. A monitoring system comprising: a deviceoutputting a light beam; a diffractive optical element creating amonitor beam from the light beam, the creating including both splittingoff a percentage of the light beam and performing further at least oneof focusing and collimating on at leapt one of the light beam, aremaining light beam not split off being transmitted through thediffractive optical element; and a detector receiving the monitor beam.20. The monitor of claim 19, wherein the diffractive optical element isa transmission diffractive deflecting a percentage of the light beam toform a deflected beam.